A razor wire-topped fence and watch tower at the US naval station in Guantánamo Bay, Cuba.
Photograph: Mladen Antonov/AFP/Getty Images

The defense secretary, James Mattis, has until end of play on Monday to produce new proposals for the future of the military detention camp at Guantánamo Bay, Cuba, including the possibility of sending new captives to be held at the controversial base.

In January, Mattis was given a 90-day deadline to carry out a review of how military captives and terrorist suspects are handled. The deadline, 30 April, formed part of Trump’s executive order that instructed officials to keep Guantánamo open, overturning Barack Obama’s aim to close it.

As part of the January order, Trump instructed Mattis to come up with new policies “regarding the disposition of individuals captured in connection with an armed conflict, including policies governing transfer of individuals to US Naval Station Guantánamo Bay”.

It is not known whether the White House or Pentagon intend to make the review public, or how quickly any new policies might be implemented. A department of defense spokeswoman, Commander Sarah Higgins, said they were in the “final stages of providing a recommendation to the White House” regarding the review, but she did not specify when that would happen nor whether it would be kept secret.

Lawyers acting for Guantánamo detainees will be keeping a beady eye on what the review says about the future of the camp, as well as looking out for any disagreements within the administration.

“It’s going to be interesting to see what the defense secretary says, both in terms of what he recommends and how those recommendations are received by the White House,” said Wells Dixon, a lawyer with the New York-based Center for Constitutional Rights who represents two Guantánamo inmates and has been counsel to dozens more.

The big question for Guantánamo watchers is whether Mattis will be at loggerheads with key White House figures, notably Trump’s new hawkish national security adviser, John Bolton, with whom his relations are already reportedly strained. Mattis has made it clear over a long military career, including his stint between 2010 and 2013 as commander of US Central Command (Centcom), that he is opposed to the involvement of the US military in long-term detention.

Bolton has publicly praised the president’s call for the camp to stay open. When the decision was announced by Trump at his first State of the Union address in January, Bolton said he supported the idea of dealing with terrorists according to a “war paradigm”.

How stupid is the president of the United States, and will he threaten US military operations against Isis?

Wells Dixon, Center for Constitutional Rights

Bolton has made little attempt to disguise his contempt for previous approaches to national security issues in the White House and his desire to forge his own path. Within days of starting as Trump’s top national security adviser he fired the president’s homeland security adviser, Tom Bossert.

There are 41 detainees at Guantánamo, which was opened in 2002 as part of the response to the 9/11 terrorist attacks as a place to hold fighters captured in Afghanistan outside the normal legal restraints of civilian courts. Of the current population, five inmates have been cleared by the US government under its own security rules as being safe to transfer out of Guantánamo.

What happens to those five will be one of the most controversial aspects of the Mattis review.

Dixon said that given the defense secretary’s record, he expected the review to recommend the five detainees should be shipped out of Cuba to other countries. But the lawyer added: “I would not expect the White House to accept that recommendation, for purely political reasons.”

Another highly contentious question is whether new captives could be brought to the camp. In his State of the Union speech, Trump suggested that Isis and al-Qaida captives could find themselves sent to Guantánamo.

However, a renewed pipeline to the Cuban base would be fraught with difficulty. Not only does it already cost $440m to run the camp, but bringing Isis fighters to Guantánamo could also present the US military with major legal challenges.

Jim Mattis talks with John Bolton in the Cabinet Room at the White House. Photograph: Kevin Lamarque/Reuters

US operations against Isis in Syria and Iraq have so far been conducted under 2001 legislation passed in the wake of 9/11 authorizing the use of military force against the terrorists responsible for the New York and Pentagon attacks. That justification is highly disputed, but nobody has been able to challenge it in the courts for lack of legal standing.

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Were an Isis leader brought to Guantánamo, he would conceivably have the ability to challenge the legal justification for US military operations on grounds that the militant organization did not even exist at the time of the 2001 attacks. Nor does Isis have any links to al-Qaida.

As a result, it is unlikely that the defense secretary would recommend bringing Isis fighters to the camp for fear that by doing so the entire legal basis of US operations in Syria and beyond could be called into question.

Again, lawyers for the Guantánamo detainees are prepared to see internal tensions within Trump’s top team. Wells Dixon said he expected Mattis to oppose sending any newcomers to the prison for fear of undermining the legal justification for the war in Syria, though that advice might not be accepted by Trump.

Dixon said: “The open question is this: how stupid is the president of the United States, and will he threaten US military operations against Isis by bringing an Isis suspect to Guantánamo?”